Automotive Air Conditioning Information Forum

The gauges that come with those charging kits (typically for indicating only the low-side pressure) aren't known for accuracy. Some of them give you just colors on the dial--like green, yellow, and red--instead of numerical pressures, making it even harder to determine the actual readings. For that you'll need a high-quality manifold gauge set, at a minimum, like those offered by this forum's sponsor.

But before investing in the proper tools and trying to measure and interpret anything, I advise you to take the vehicle to a credible mobile AC shop to have the refrigerant charge recovered and weighed. That's really the only way to determine how much R-134a is in it now. With that knowledge in hand, the shop can proceed to evacuate and recharge the system to spec, check for leaks, and evaluate the pressures and system performance from there.

Yes, you still could have a blend door, actuator, or other issue compounding the problem. But unless we know for certain that the system is free of air and moisture and has the correct charge, we're really mostly guessing at the latter and an informed diagnosis.

Many thanks for that comprehensive and thoughtful summary John and, as I said in my original post, the next step is to hand over to the pros - exactly as you're advising. Thanks in fact to all who have taken the trouble to provide their thoughts; I now have a rather better understanding of the many and varied complexities involved.

Indeed, mobile AC can be very tricky to diagnose and repair.

I'll repeat what someone else mentioned, and that is to be mindful about what refrigerant you intend to use. Some brands contain additives and sealants, the latter of which can clog-up a system and damage it beyond repair, requiring the costly replacement of components. Professional AC shops know this and will refuse to work on a system containing sealant because it damages their expensive equipment as well. Leak-detecting dye is usually okay, but I'd be wary of anything else in the can other than pure refrigerant.

Let us know the outcome of your project.

Noted. I'll certainly let you know what the professional diagnosis and eventual outcome is.

Rather belatedly (coronavirus complications, natch) but as promised, I'm posting to let you all know the outcome of the situation. Quite simply, the compressor was shot, hence no working aircon, but this in no way accounts for the sky-high pressure reading on the low side port. Having now tried the pressure gauge on two other cars (with working aircon) I got the same worryingly high readings of nearly 100psi, well into the red sector on the dial, so my conclusion is that I've been sold a duff bit of kit which I now intend returning for a refund. Again, many thanks for your earlier thoughts and observations and, despite what I see on Youtube, henceforth I will leave all such work to the pros!

Glad that you have the system running—just in time for summer—and that you decided to return the charging kit. Visit again should you need us.

Thank you John. Will do.